1. Field of the Invention
The present invention relates to the production of integrated circuits. More particularly, the invention relates to the removal of an edge bead of spun-on nanoporous dielectric silica coatings on substrates which are used in the production of integrated circuits.
2. Description of the Prior Art
In the production of integrated circuits, several methods have been developed to deposit a dielectric coating onto a substrate. One common method is spin-coating of dielectric compositions on such substrates. Spin-coating involves deposition of a liquid composition onto a spinning substrate to thereby uniformly coat a surface of the substrate. Spin-coating has been widely used in the formation of nanoporous dielectric silica coatings on substrates such as semiconductor substrates.
Nanoporous dielectric silica coatings can be formed by depositing a liquid alkoxysilane composition onto a spinning substrate to thereby coat the substrate. The coating is typically polymerized, condensed, and cured to form a nanoporous dielectric silica coating on the substrate.
In processes such as the manufacture of integrated circuits, a liquid coating on a spinning substrate tends to spread to the substrate edge, thereby forming an edge bead. Edge beads formed from brittle substances such as silica tend to shatter upon subsequent processing, especially when machinery comes in contact with the edge of the substrate. This leads to the generation of tiny particles which contaminate further processing of the integrated circuits. Ideally, a 3-5 mm wide area at the edge of a substrate should be free of coating material to avoid this particle contamination.
Various techniques are known in the art to remove the edge bead from the substrate during or after spinning. One method of edge bead removal (EBR) is by the use of solvents such as alcohols to dissolve the coating on the substrate's edge. However, since alkoxysilane compositions often contain low volatility solvents, they tend to remain liquid after deposition onto the substrate. If a solvent such as an alcohol is used to remove the edge bead, the wet alkoxysilane composition can still spread to the edge of the substrate, similar to ink spreading on wet paper. Preventing this further problem would require cleaning a wider edge area, thus reducing the usable surface area of the substrate.
U.S. Pat. No. 4,510,176 discloses a process for removing photoresist compositions from the edge of a semiconductor substrate. A stream of a solvent is first directed onto the edge of a spinning, photoresist coated semiconductor substrate. After the coating material is dissolved, a gas is discharged onto the spinning substrate to carry loose fluid and/or debris away from the center of the substrate. U.S. Pat. No. 4,732,785 discloses a process for edge bead removal of spun-on glass compositions. A stream of a solvent is directed onto the edge of a spinning substrate to thereby remove an edge bead which contains SiO.sub.2. The substrate is spun at a first rotational speed and then a second lower rotational speed to thereby gradually remove the edge bead during deceleration.
It would be desirable to develop an edge bead removal method which results in the formation of a spin-coated nanoporous dielectric silica coating on a substrate with an uncoated edge. The present invention offers a solution to this problem. In one embodiment of the invention the deposition of a surface hydrophobizing agent, such as hexamethyldisilazane (HMDZ), onto a spinning substrate results in the clean removal of an alkoxysilane composition from a substrate edge. The surface hydrophobizing agent causes the substrate surface to become hydrophobic such that it repels alkoxysilanes and low volatility solvent materials in the edge areas. In another embodiment of the invention an edge bead can be cleanly removed by dispensing a solvent for the coating material onto the coated edge and spinning the substrate first at a relatively low speed and then at a relatively high speed to first partially remove and then substantially totally remove the composition from the edge of the substrate.